This is an open-label, single-arm pilot study evaluating the antitumor activity and safety of niraparib as neoadjuvant therapy in participants with Human epidermal growth factor receptor 2 (HER2)-negative and breast cancer susceptibility gene mutant (BRCAmut) localized breast cancer (primary tumor >=1 centimeters [cm]). Breast magnetic resonance imaging (MRI), breast ultrasound, and tumor core biopsy will be performed at the screening (Days -28 to -1). Participants will receive niraparib (200 milligrams [mg] orally [PO]) treatment daily for 28 days (Cycle 1) and then will undergo breast ultrasound at the end of Cycle 1 on Day 28. Based on breast ultrasound reports, the participants will either discontinue the study (disease progression) or will continue niraparib treatment (complete response [CR], partial response [PR] or stable disease [SD]) for an additional cycle (Cycle 2). A breast MRI and breast ultrasound will be performed at the end of Cycle 2. Approximately 21 participants will be enrolled in this study and the study duration will be approximately 2 years.

Inclusion Criteria:

• Participants age >= 18 years old.

• Participants with a deleterious or suspected deleterious breast cancer susceptibility gene 1 (BRCA1) or BRCA2 mutation (germline or somatic) may be enrolled into the study based on either local or central laboratory testing of BRCA status.

• Histologically-confirmed HER2-negative localized breast cancer by core biopsy.

• Primary operable, non-metastatic invasive carcinoma of the breast, confirmed histologically by core biopsy. Fine-needle aspiration is not sufficient. Incisional biopsy is not allowed. In participants with multifocal and/or multicentric, the largest lesion should be measured. Both unilateral and bilateral breast cancer are allowed.

• Primary tumor size >=1cm.

• Measurable disease by breast ultrasound and MRI.

• Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1.

• Adequate organ function defined as:

  1. Absolute neutrophil count (ANC) >=1500 per microliters (/μL).
  2. Platelets >=100,000/μL.
  3. Hemoglobin >=9 grams per deciliter (g/dL).
  4. Serum creatinine <=1.5*upper limit of normal (ULN) or calculated creatinine clearance >=50 milliliters per minute (mL/min) using Cockcroft-Gault equation.
  5. Total bilirubin <=1.5*ULN except in participants with Gilbert's syndrome. Participants with Gilbert's syndrome may enroll if direct bilirubin <=1.5*ULN of the direct bilirubin.
  6. Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) <=2.5*ULN.

• Participants must have recovered to Grade 1 toxicity from prior cancer therapy (a participant with Grade 2 neuropathy or Grade 2 alopecia is an exception to this criterion and may qualify for this study).

• Participant able to take oral medications.

• Participant meets the following criteria:

  1. Female participant (of childbearing potential) is not breastfeeding, has a negative serum pregnancy test within 72 hours prior to taking study drug, and agrees to abstain from activities that could result in pregnancy from Screening through 180 days after the last dose of study drug, or is of non-childbearing potential.
  2. Female participant is of non-childbearing potential (other than medical reasons) as defined:

  i) >=45 years of age and has not had menses for >1 year. ii) Amenorrheic for <2 years without a hysterectomy and oophorectomy and a follicle-stimulating hormone value in the postmenopausal range upon the screening evaluation.

iii) Has undergone post hysterectomy, bilateral oophorectomy, or tubal ligation. Documented hysterectomy, oophorectomy or tubal ligation must be confirmed in the medical records, otherwise the participant must be willing to use 2 adequate barrier methods throughout the study starting from the screening visit through 180 days after the last dose of study drug. Information must be captured appropriately within the site's source documents.

c) Male participant agrees to use an effective method of contraception starting with the first dose of study therapy through 120 days after the last dose of study therapy.

• Able to understand the study procedures and agree to participate in the study by providing written informed consent.

Exclusion Criteria:

• Prior anti-cancer therapies for current malignancy.
• Known evidence of distant metastasis. Staging studies are not required. The decision to pursue staging studies is at the discretion of the treating clinician, based on the participant's clinical and pathological findings consistent with standard guidelines.
• Known hypersensitivity to the components of niraparib components or their formulation excipients.
• Major surgery within 3 weeks of starting the study or participant has not recovered from any effects of any major surgery.
• Poor medical risk due to a serious, uncontrolled medical disorder, non-malignant systemic disease or active, uncontrolled infection. Examples include, but are not limited to, uncontrolled ventricular arrhythmia, recent (within 90 days) myocardial infarction, uncontrolled major seizure disorder, unstable spinal cord compression, superior vena cava syndrome, uncontrolled hypertension, active uncontrolled coagulopathy, bleeding disorder, or any psychiatric disorder that prohibits obtaining informed consent.
• History or current evidence of any condition, therapy, or laboratory abnormality that might confound the results of the study, interfere with the participant's participation for the full duration of the study drug, or is not in the best interest of the participant to participate.
• Participant is pregnant or breastfeeding, or expecting to conceive children within the projected duration of the study drug or within the 180-day period after the last dose of study drug.
• Immunocompromised participants.
• Known active hepatic disease (Hepatitis B or C).
• Prior treatment with a known PARP inhibitor.
• Other active malignancy that warrants systemic therapy.
• Known history of myelodysplastic syndromes (MDS) or Acute myeloid leukemia (AML).

GSK will assess requests from qualified researchers for anonymized individual patient-level data (IPD) and related study documents. Data sharing is subject to certain criteria, conditions, and exceptions. For further information, refer to https://www.gsk-studyregister.com/About\_GSK\_Patient\_Level\_Data\_Sharing\_Final\_13July2023.pdf

Anonymized IPD is made available within 6 months of publication of primary, key secondary and safety results for studies in product with approved indication(s) or terminated asset(s) across all indications.

Anonymized IPD is shared with researchers whose proposals are approved by an Independent Review Panel and after a Data Sharing Agreement is in place. Access is provided for an initial period of 12 months but an extension may be granted, when justified, for up to 6 months.

Up to 1 year

TEAEs and SAEs were collected in the Safety Population which comprised of participants who received at least one dose of study medication.